In FakeStatsd, use select() rather than poll()

The poll() system call is not available on all systems (like OS X, for
example), causing the test suite to fail. This change replaces poll()
with select(), which is more portable. With this change the test suite passes
under OS X.

Change-Id: I3ad716b127c8781d8dbf870b0f0c0e1162f264d1
Reviewed-on: https://review.openstack.org/28126
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins